embryo screening, procedure (see genetic testing) in which a single cell is removed from an embryo two or three days after it has been conceived through in vitro fertilization and tested for genetic abnormalities. At this stage the embryo consists of about eight genetically identical cells. The embryo itself is unaffected and continues to grow while the selected cell's genes are replicated and tested for genetic defects. Embryo screening, also known as embryo biopsy, preimplantation genetic diagnosis, and preimplantation genetic screening, permits genetic testing to occur before an embryo is implanted into the womb. Such procedures are used when an inheritable disease or a genetic predisposition to a disease is carried by or exhibited in one or both parents. The techniques have also been used to improve the success of in vitro fertilization in general by determining if an embryo has any abnormalities that might lead to a miscarriage after the embryo is implanted.

Embryo screening has been used successfully to screen for such diseases as cystic fibrosis and Tay-Sachs as well as for genes that predispose a person to breast or colon cancer. The procedure can also be used to determine the gender of the embryo. Given this ability, X-chromosome-linked diseases that manifest only in males (hemophilia, for example) can be "screened" by implanting only female embryos (which will carry but will not develop the disease). Another variation that involves examining the sample to make sure it has the correct number of chromosomes is used for women over 35 (women past that age being statistically more likely to give birth to babies with Down syndrome and other chromosomal defects). Embryo screening has also been used to select a child who is a compatible donor for a sibling with a life-threatening disease such as certain anemias and leukemias.

See also amniocentesis; birth defects; chorionic villus sampling.